Water purification systems are used to provide high quality reagent grade water for various applications, including the field of scientific testing and analysis. Many of these applications require that the total organic carbon content of the water be on the order of 10 parts per billion or less ASTM. Type I water is the highest purity and is used for high performance liquid chromatography, atomic absorption spectrometry, tissue culture, etc. Type II water is less pure and may be used for hematological, serological, and microbiological procedures. Type III water is suitable for general laboratory qualitative analyses, such as urinalysis, parasitology and histological procedures. Two prior systems for purifying water are disclosed in U.S. Pat. Nos. 5,397,468 and 5,399,263, each assigned to the assignee of the present invention. The disclosure of each of these patents is hereby incorporated by reference herein.
Purified water dispensing systems that currently provide automatic controlled dispensing of water do so using a timed dispense technique. This is accomplished by electronically controlling a solenoid valve and holding the solenoid valve open for a user-programmed time period. The user sets this time based on the amount of water they wish to dispense from the system. The user determines a relationship between dispensing time and flow rate for their specific system and operating conditions. Another method of controlling the dispensed amount of purified water involves manually opening a valve with the system pump shut off. Actuation of a switch in the valve initiates the pump when the valve is open. The pump remains energized for a time programmed by the user. When the time has expired, the pump is turned off by the control system. The manual valve remains open until the user returns to the system to close this valve. One significant drawback to this method is that the manual valve may remain open for some time until the user returns to shut it off. The main reason for using this method is to provide a manner of dispensing water into a larger vessel without holding a remote operating valve open for a long period of time and without running the water out of the vessel. Another drawback to both of these prior methods relates to the accuracy of the volume dispensed when relying on a user-defined relationship between dispensing time and dispensed volume. If the time value entered by the user is too long, the vessel being filled may run over. Generally, if the time value is incorrect by being either too long or too short, the user must manually correct the dispensed amount of water by removing water from the vessel or manually filling the vessel to the required amount. This, of course, defeats the purpose of having an automatic dispense control. The relationship between dispensed volume and dispensing time will also vary for any given system, depending on the pressure at the inlet of the system, the voltage on any pump associated with the system, the condition of the filters and membranes, among other factors.
In light of these and other problems in the art, it would be desirable to provide a water purification system having an accurate and automatic manner of sensing and, preferably, controlling the volume of water discharged from the system.